Can sufficient means of subsistence be shown in the form of a declaration of parents about the obligation to send money to the child?

NO. Documents that may confirm the possession of funds for subsistence by the student are a traveler's cheque, a certificate of the credit card limit issued by the bank that issued the credit card, a certificate that a foreigner or a person who is dependent on the foreigner has cash in the bank or a cooperative savings and credit union with its registered office in the territory of the Republic of Poland or another Member State of the European Union, a Member State of the European Free Trade Association (EFTA) - a party to the Agreement on the European Economic Area or the Swiss Confederation, a document confirming the granting of a domestic or foreign scholarship, a certificate of employment and the amount of earnings of the foreigner or the person on whom the foreigner is dependent.